One type of gas discharge laser used in photolithography is known as an excimer laser. An excimer laser typically uses a combination of a noble gas, such as argon, krypton, or xenon, and a reactive halogen gas such as fluorine or chlorine. The excimer laser derives its name from the fact that under the appropriate conditions of electrical stimulation and high pressure, a pseudo-molecule called an excimer (or in the case of noble gas halides, an exciplex) is created, which can only exist in an energized state and can give rise to laser light in the ultraviolet range.
Excimer lasers are widely used in high-resolution photolithography machines, and are thus one of the critical technologies required for microelectronic chip manufacturing. Current state-of-the-art lithography tools use deep ultraviolet (DUV) light from the KrF and ArF excimer lasers with nominal wavelengths of 248 and 193 nanometers respectively.
While excimer lasers may be built with a single chamber light source, the conflicting design demands for more power and reduced spectral bandwidth have meant a compromise in performance in such single chamber designs. One way of avoiding this design compromise and improving performance is by utilizing two chambers. This allows for separation of the functions of spectral bandwidth and pulse energy generation; each chamber is optimized for one of the two performance parameters.
Such dual-gas-discharge-chamber excimer lasers are often called Master Oscillator-Power Amplifier, or “MOPA,” lasers. In addition to improving the spectral bandwidth and pulse energy, the efficiency of the dual chamber architecture can enable the consumable modules in MOPA lasers to reach longer operational lifetimes than their counterpart modules in single chamber light sources.
In each chamber, as the light source discharges energy across its electrodes to produce light, the halogen gas, fluorine in the case of ArF or KrF lasers, is depleted. This causes a decrease in the laser efficiency which is seen, for example, as an increase in discharge voltage required to create a given desired pulse energy. Since the discharge voltage has an upper limit determined by physical constraints of the hardware, steps must be taken to replenish the lost fluorine so that the voltage remains below this limit and the laser continues to function properly.
One way to do this is with a full replenishment of the gas in the chambers, called a refill, where all of the gas is replaced while the laser is not firing to return the gas content in the chamber to the desired mix, concentration and pressure. However, refills are extremely disruptive as the laser must be shut off during the refill process, and thus the lithographic exposure of semiconductor wafers must also be paused in a controlled manner at the same time and then restarted when the laser is again operational to avoid improper processing of the wafers. For this reason, it is typical to refill both chambers at once to save time, although this is not necessary.
The need for a refill can depend on several complex and often unpredictable variables, including the light source firing pattern and energy, the age of the light source modules, and others that will be familiar to those of skill in the art. For this reason, refills are typically done on a regular schedule, which ensures that the light source operation will never suffer unanticipated interruption due to the light source reaching its operational limit. Such a regular schedule generally yields very conservative upper limits on the time between refills, such that some users of the light source operating at low pulse usages might be able to wait for a much longer period of time between refills than is provided by the simple schedule.
Given the demands of increased throughput and light source availability, efforts have been made to minimize light source stoppage for refills. One way of doing this is by performing a partial replenishment of the gas in the chambers, known as an inject, rather than a full refill. As long as the laser is able to continue to operate within certain parameters, it is not necessary to shut the laser down for the inject, and thus processing may continue during the inject process.
Another factor that decreases efficiency is the ageing of the laser; older excimer lasers are in general less efficient than newer ones. However, it can be difficult to separate the effects of ageing from the effects of fluorine concentration that is not optimal, particularly when the control of the fluorine is not precise.
A number of prior methods and systems have been described for managing injects, including, for example, how to determine when an inject should occur and the amount of halogen gas to be provided by the inject. See, for example, U.S. Pat. Nos. 7,741,639 and 7,835,414, owned by the assignee of the present application. However, such prior art still requires that a refill be done at some point to keep the laser within operating parameters.
Further, many of these prior art techniques do not have adequate control of the fluorine concentration to be able to tell whether a given loss of efficiency is due to the fluorine level or to the ageing of the laser. As a result, those using such prior art techniques generally assume that any loss of efficiency is due to a fluorine problem and attempt to adjust the fluorine to compensate, and thus do not compensate for the ageing of the laser in any way.
It would be desirable to have a method that allows a laser to operate within acceptable parameters for a longer period of time using only injects rather than refills to replenish the gas in the chambers, so that the laser need not be shut off during the refill procedures. It would also be desirable to be able to compensate for the ageing of the laser as part of such a method using only injects rather than refills.